This invention relates in general to signal controlled variable frequency oscillators and in particular to those suitable for fabrication in integrated circuit form.
Integrated circuits (IC's) comprise a number of semiconductor devices formed on a single substrate or monolithic "chip". Their use in electronic circuit applications is growing because of their size, economy and reliability, especially in small signal processing circuitry. Large numbers of transistors, resistors and diodes, as well as small capacitors may readily be formed on a chip which is typically 2,000 to 10,000 square mils in area. The chip circuitry has terminals for signal coupling and bringing in operating potentials. Additionally, inductive circuit elements, large capacitors and other external components are connected to selected portions of the chip circuitry through terminals, which are connected to the chip circuitry by conductive paths terminating in bonding pads located near the chip periphery.
The monolithic chip is packaged, or encapsulated within an insulating container and connections are made, generally with gold filament-like leads, between the bonding pads and corresponding terminals. Once encapsulated, the IC package is an insulated, sealed capsule having a number of exposed terminals for making connections between external devices and those within the chip. The number of exposed terminals which can be accommodated is controlled by practical limitations of integrated circuit packaging.
A commonly used integrated circuit package in consumer electronics is the dual-in-line type in which a flat, elongated, molded capsule houses a monolithic chip and supports two parallel rows of terminals along its sides, giving the package a caterpillarlike appearance. The package generally has from 12 to 24 terminals. Configurations with more than 24 terminals have disadvantages, such as difficulty of insertion into mating sockets without terminal damage and much higher probability of defective terminal bonding connections.
IC economics is largely predicated on high volume production, and consequently, there is great emphasis placed on standard packaging. Often, desirable circuit configurations are not implemented for lack of a terminal. Similarly, a large number of devices may be in use and servicing considerations dictate that succeeding devices be compatible and interchangeable.
Since it is difficult to make integrated circuit signal coupling capacitors, the requirement of minimizing external components (and thereby terminals) for efficient large scale integration dictates that wherever possible internal stages be directly coupled within the integrated circuit rather than via external coupling components.
Signal controlled variable frequency oscillators, which are often used in the types of electronic systems sought to be integrated, typically include a resonant circuit, consisting of conventional elements, for establishing the oscillator's natural frequency and a control signal responsive variable resonance network for changing the oscillator frequency. The variable resonance network may include variable reactance elements, such as variable capacity diodes, or variable conduction elements, such as transistors, in conjunction with conventional non-integratable inductive and capacitive elements. Thus, conventional signal controllable oscillators include many nonintegratable elements which are connected to the IC chip by terminals.
The difficulty of fabricating integrated semiconductor devices to external standards is well known. Likewise, the ease of fabricating IC semiconductor devices of similar characteristics on the same chip is also well known. Consequently, the basic building block of integrated circuitry is the differential amplifier (having paired configurations of transistors of like characteristics) and wherever possible it is desirable to process signals in differential or balanced form.
The signal controllable variable frequency oscillator of the invention produces a balanced variable frequency output, for DC coupling to other integrated stages, and is frequency controllable without external resonant circuitry.